Method of making porous bearing surfaces



Aug. 24, 1948.l F. R. HENsEL. 2,447,930

METHOD OF MAKING POROUS .BEING SURFACES Filed Jan. 29, 1945 2 Sheets-Sheet 1 INVENTOR.

Patented Aug.` 24, 1948 Ma'rnon or MAKING roaous BEARING summons Franz R. Hensel, Indianapolis,

Ind., assignor to P. R. Mallory & Co., Inc., Indianapolis, Ind., a corporation of Delaware Application January Z9, 1945, Serial No. 575,159

(Cl. 2li-149.5)

4 Claims. l This invention relates to a process for making porous or impregnated metal layers. Thi-s application is a continuation in part of my appli# cation Serial No. 502,740, filed September 17, 1943, for Porous metal layer.

An object of the invention is to improve the methods of making porous metal bodies.

Another object is to provide a metal body with a surface area of porosity and method of making same.

Another object is to improve manufacture.

A further object is to provide improved metal bodies such as electrolytic condenser electrodes, bearings, filters and the like.

Other objects of the invention will be apparent from the description and claims.

In the drawings:

Figure l illustrates a vacuum apparatus for making a porous metal body;

Figure 2 shows a method of impregnating the body;

Figure 3 illustrates a step in another impregnation method; l

Figure 4 illustrates a bearing of modied form;

Figure 5 shows a section through a. porous metal sheet;

Figure 6 illustrates a vacuum apparatus for diffusion and evaporation for producing porosity in metal bodies; and

Figure 7 shows a sectioned bearing with a surface area of porosity; y

Figure 8 illustrates another vacuum apparatus for evaporation for producing porosity in metal bodies.

The apparatus shown represents experimental set-ups schematically illustrative of the processes of. the invention.

This invention is directed to forming porosity in metallic members by distributing one metal in another and then removing an appreciable percentage of one of the metals.

An illustrative feature of the present invention resides in. producing an alloy or metal composition containing a solid metal of low boiling point, such as zinc or cadmium, and subsequently evaporating the low boiling metal to leave a porous metal body. An important feature is that this alloy or composition may be formed by diffusion. It is also contemplated that the alloy may be worked, rolled and formed into the desired shape prior to evaporation. It is further contemplated that the pores produced by evaporation may be subsequently impregnated with another metallic or non-metallic material.

bearings and their 4ents have been taken out An important application of the process is in the manufacture' of bearing materials.

The preparation of copper-lead bearing al loys in the form of a bimetallic strip has presented fabrication diiiiculties which have not been overcome as yet. A large number of patwhich describe methods of producing such strip, starting with powdered metals followed by an impregnation process. The structure, however, is extremely weak and therefore the bearings have a tendency to segregate in the bond, thereby weakening it.

According to the present invention a steel-brass overlay metal may be produced by well known methods such as brazing, fusing the brass directly to the steel, plating, spraying, hot rolling the metals together, etc., and is rolled down to practically finished dimensions. It may then appear as a clad metal sheet or a lined bearing shell or half-shell. The thickness of the brass may be between a few thousandths of an inch and 25 to 50 thousandths.

The composite metal body thus formed ls placed in a vacuum chamber 9 as shown in Figure 1, where bearing half-shells I0 and a piece of overlay metal sheet I I are shown in the chamber preparatory to treatment. The vacuum pump is started to evacuate the chamber through pipe I2. The bodies are then heated, as by resistance heating element I3, to the vaporization temperature of the zinc at the reduced pressure but below the melting point of the brass, for example 700-850 C. This results in the vaporization of the zinc providing a honeycomb structure of extremely fine micro-porosity.

For use as a bearing the porous material may be used as such or the pores are filled with an anti-friction material such as oil or lead, indium, thallium, Babbitt, or other lubricant materials.

Figure 2 shows a method for impregnating with lead or other metal. The bearing blanks I I having the porous copper surface resulting from evaporation of the zinc from the brass layer are placed in chamber I4 and covered with a bath I5 of molten lead, or other lubricant. A heater IB maintains the bath temperature, and rack I"I holds the part-s under the bath. Alternate vacuum and pressure are applied to the bath through pipes I8 and I9, respectively, to remove gases and promote entry of the lubricant into the pores. Itis contemplated that the impregnation may be carried out in the evaporation chamber, if desired.

The original bond between the brass and the steel backing is not adversely affected by the vaporization process. The porous structure itself is extremely strong due to the fact that it is created from a previously fused alloy, the structure of such alloy being of the solid solution type where the crystal of alternate atoms of copper and zinc for instance.

It is contemplated that silver, gold. platinum, palladium and aluminum base bearings may also be made by this process, substituting sliverzinc or aluminum-zinc alloys, for example, for the brass. Other low boiling metals may in some cases be substituted for the zinc, such as cadmium, mercury, phosphorus, sulphur, selenium, rubidium, potassium, arsenic, cesium.

In Table 1 the vapor pressures of zinc and cadmium are given as a function of temperature while in Table 2 the vapor pressures of such metals as copper, silver and aluminum are shown for comparison.

TABLE 1 Vapor pressure of zinc 256.8I C., .00012 mm. solid 419.4 C., .15 mm. melting point 588.8 C., 9.051 mm. liquid 720.4 C., 81.42 mm. liquid 836.0o C., 356.2 mm. liquid 905.0 C., 760 mm. liquid Vapor pressure of cadmium l98.7 C., .00027 mm. solid 302.9 C., .102 mm. melting point 570.8 C., 51.81 mm. 'liquid 706.7 C., 371.3 mm. liquid 765.9 C., 760.0 mm. liquid TABLE 2 Vapor pressure of silver 1178 C., .144 mm. liquid 1435 C., 3.9 mm. liquid 1660 C., 102 mm. liquid 1780 C., 263 mm. liquid 1955"' C., 706 mm. liquid Vapor pressure of copper 1875" C., mm. liquid 1980o C., 100 mm. liquid 2215 C., 300 mm. liquid 2310 C., 760 mm. liquid Vapor pressure of aluminum 1203 C., .01 mm. liquid 1400 C., .23 mm. liquid 1700 C., 10 mm. liquid 1964 C., 100.6 mm. liquid 2270 C., 760 mm. liquid It will be noted that the boiling points of zinc (905 C.) and cadmium (765.9 C.) at atmospheric pressure are below the melting point of copper 1083 C.) although some of the brasses -melt at lower temperatures, even as low as 905 C. The melting point of silver (960.5 C.) is closer to the boiling point of zinc and that of aluminum (669.7 C.) is below the boiling points of both zinc and cadmium. Vaporization should be carried on at temperatures below the melting points of the starting alloys and the residual metal. It is clear that a vacuum process is preferable as it permits fairly rapid volatilization of the zinc or cadmium at lower temperatures. The degree of vacuum need not be extremely high as effective results can be obtained at 1A to 1/2 atmospheric pressure and even at higher pressures. In fact with the higher mlling lattice structure is composed point alloying metals such las copper it is possible to work at atmospheric pressure by carefully regulatingthe temperature to maintain it as high as possible without melting the layer. As the zinc and cadmium is removed it is also possible to raise the temperature toward the end of the volatilization period. Vaporization should preferably be carried out in a neutral or reducl ing gas atmosphere to prevent oxidation of the porous layer.

Since the zinc and cadmium have substantial vapor pressures even below their boiling points, under the pressures used it is not always necessary to reach the boiling points of these metals. A substantial removalof these metals is obtained below their boiling points if the time of treatment is prolonged.

It is evident that there is a considerable difference of vapor pressures for these two different classes of metals for a given temperature. Comparing cadmium and silver for instance, we find that cadmium will have a strongvtendency to vaporize at 600 C. while the vapor pressure of silver is so low at 600 C. that it cannot be measured. It would be necessary in the case of silver to go to l800 C. to obtain a similar rate of vaporization.

These tables also indicate the degree of vacuum necessary to cause vaporization. The higher the vapor pressure the lower may be the vacuum to accomplish vaporization. The tables therefore show that more vacuum equipment is required if the vaporization process is to be carried out at lower temperatures.

In the case of silver I have found that silver cadmium alloys are particularly useful since they can be worked easily and since cadmium has a high vapor pressure facilitating evaporation.

Another method of impregnating comprises applying a layer 20 of lead or other lubricant metal over the porous copper or other porous metal layer 2| which is bonded to backing layer 22 of steel, nickel or other strong backing material as shown in Figure 3.A Layer 20 may be sprayed onto layer 2i. electroplated, or merely laid on as a sheet or in the form of powder. The assembly is then heated, preferably in a neutral or reducing atmosphere to bring about impregnation and diffusion of the metal of layer 20 into layer 2i.

The amount of porosity can be controlled by selecting the proper amount of low boiling point elements in the alloy. In the case of brasses we have rather wide limits within which the invention may be practiced. It is possible to use zinc contents as low as several per cent and as high as 40 per cent. If it is desired to obtain a continuous network of micro-porosity the limits of zinc will be in the neighborhood of 15 to 20%. While a continuous network of pores is desirable for self lubricating bearings a discontinuous porosity is preferred for high speed and high load bearing applications.

Instead of using binary alloys of only one low boiling point constituent complex alloys containing several low boiling point metals can be used such as silver base alloys containing both zinc and cadmium,

The porosity of the alloys can also be controlled by properly correlating thickness of sheet, temperature at which evaporation is carried out, time, degree oi' vacuum. The evaporation process relies upon diffusion of the low boiling point metals from the center of the sheet to the surface where evaporation takes place. While in some cases it is desired to eliminate the low boiling point metal entirely there are other cases where only partial elimination is required.

During the evaporation process the low boiling metal is eliminated from the surface layers first. It is therefore possible to stop the evaporation process at an intermediate stage to leave a porous surface layer `backed by the original alloy, such as brass. The porous surface layer may, for certain bearing applications, be impregnated with a lubricant such as oil, lead or thallium.

Figure 4 shows such a bearing body comprising a steel backing 23, an intermediate brass layer 24 and a bearing surfacelayer 25 of porous copper impregnated with lead or the like.

Figure 5 shows a cross section of a porous sheet 26 formed by evaporating the zinc or cadmium from an alloy sheet to leave it porous throughout its thickness.

Such a sheet can be used in bearing manufacture by brazing it to a backing, electroplating a backing onto the sheet or by other methods. It may be formed of porous aluminum, copper, silver or other bearing metals.

In the making of porous Sponges of copper or silver by evaporating alloy materials from them at elevated temperatures in a vacuum the pore size of the sponge is extremely small.

Alloys of copper-zinc, silver-cadmium and silver-zinc containing 30% of the volatile component have been cast as ingots. These were reduced by hot rolling, followed by cold rolling to about 0.020", pickled and the more .volatile element evaporated. This evaporation was carried out in an evacuated furnace, samples being run at the following temperatures:

Degrees F. Copper-zinc 1500 and 1380 Silver-cadmium 1380 and 1200 Silver-zinc 1150 and 1000 At each temperature, several samples were evaporated for periods of 6, 24, and 48 hours. With the copper-zinc alloy evaporation for a period of 48 hours at a temperature of 1500 F. and at 4 mm. pressure substantally all the zinc was removed.

In the course of the investigation a considerable number of alloys were prepared. A few examples are cited below:

Copper, 69.0%; zinc, 30.9%; cast. 1010 C. Silver, 68.7%; cadmium, 31.2%; cast. 900 C. Silver, 69.1%; Zinc, 20.4%; cast. 780 C.

The ingots were cast and reduced by hot rolling as follows:

Copper-zinc rolled to .028-.029 at 1200 F. Silver-cadmium rolled to .025-.026 at 900 F. Silver-zinc rolled to .028-.029 at 1200 F.

The samples were annealed at '150 F. The copper-zinc was pickled iu potassium dichromate and sulphuric acid in water solution; the two silver alloys were both pickled with 5% nitric acid. The copper-zinc and silver-cadmium alloys were cold rolled and the silver-zinc was hot rolled, all to 0.020".

The evaporation furnace consists of a Vitrosil tube 21 (see Figure 8) stoppered at the ends and evacuated. This tube is encased by another Vitrosil tube 28 and the intervening space between the two tubes is kept filled with hydrogen gas whichinsures that any gas which enters by diffusion through the tube walls will be of a reducing nature.

' mens are cooled under vacuum. In some cases sage through to reduce oxidation the specimens are heated for a short period in hydrogen to about 750 F.

In one experimental set-up the Vitrosil tube 21 is about 1" I. D. and is encased in another Vitrosil tube 28 of about 2" O. D. These two coaxial tubes are mounted in a resistance heated tube 29 of Alfrax in a transite case 30. Athermocouple 3'I installed in the interior tube runs to a controller which maintains the required temperature. A metal-vapor trap 32 is inserted between the evaporatlng chamber and a pump. Between trap 32 and the pump a valve is provided for isolating the chamber and a branch line runs to a modified McLeod guage. On the other end of the chamber, a sealing valve 33 is mounted which vents the system through a drying tower 34 lled with calcium chloride.

The trap 32 is immersed in a Thermos bottle 35 and is held at low temperature by a mixture of acetone and Dry Ice. While most of the vapor released in the chamber is condensed near the cool end of the tube, a small amount is carried on through and is satisfactorily removed by this cold trap.

The incoming hydrogen gas is rst passed over copper maintained at about 900 F. in a reaction tube converting any residual oxygen in the hydrogen to water, which is then removed by pascalcium chloride at 36 and phosphorus pentoxide at 31. From this point the hydrogen enters the shrouding chamber.

For the purpose of illustration the results' on two alloys of silver, one containing 30% of cadmium, and the other 30% of zinc, are included. These two alloys were subjected to evaporation of the minor constituent. This was accomplished in an evacuated furnace. The tests were run at the following temperatures:

Degrees F. Silver-cadmium 1380 and 1200 Silver-zinc 1150 and 1000 for periods of 6, 24, and 48 hours. These evaporations were carried out at 4 mm. pressure. Others were run at 0.04 mm. at the following temperatures.

Degrees F. Copper-zinc 1500 and 1380 Silver-zinc 1000 Silver-cadmium 1380 and at 70 mm., Silver-cadmium 1380 exposure at the proper temperature cooling was allowed to take place with the furnace still under vacuum. The furnace was filled with hydrogen previous to the iirst evacuation.

Silver-zinc and silver-cadmium were evaporated in vacuums of 0.02 to 0.04 mm. and at 70 Impregnation was carried out in a large test tube. The metals were suspended by a fusible wire above a quantity of S. A. E. 30 oil which was maintained at 120 C. under a vacuum of 0.04 mm. Sufficient time was allowed by degassing the specimen, it was then dropped by electrically melting the supporting wire permitting immersion while the tube was still evacuated. The metal was allowed to remain under the oil while the vacuum was released, the atmospheric pressure thus forcing the oil into the pores.

Penetration of the oil was aided by raising the oil temperature to 250 C. after releasing the vacuum. The metals were immersed for two hours.

' The extent of voids produced by evaporation of some specific amount of the volatile constituents at various temperatures and pressures on any of these alloys is found to be relatively independent of the conditions of evaporation, but closely related to the final composition. Where the volatile component is zinc a slight increase in voids is noted for higher evaporation temperatures.

Similarly, shrinkage in the metals shows relatively small variation resulting from evaporation procedure, but some uniformity with respect to the final composition.

Evaporation is about twice as fast at a low pressure as at a high pressure.

With the silver sponge produced from silverzinc the absorption of S. A. E. 30 oil is 70 percent or more. This appears to be the result of the type of pore formation during evaporating process. Absorbency varies little for a given composition produced under varying evaporating rates. With the sponge from the silver-zinc alloy the saturation was 96.8 percent of that possible.

An alloy containing 43.5 percent zinc and 56.4 percent copper was cast into slabs and reduced by hot rolling to sheet strips 0.020 inch thick. From this stock, units were machined for evaporation, and were then annealed and pickled.

Evaporation was accomplished with a pressure of 0.04 mm., at 1500 F. for 6 and 2 4 hours; at 1380 F. for 6, 24, and 48 hours; at l200l F. for 6 and 24 hours.

The connected porosity of the sponges produced from this alloy is quite high, impregnations showing absorption of oil into nearly 80 percent of the calculated voids.

In making a sponge from beta brass the zinc was removed quite rapidly from this alloy. 'I'he sponge from beta can be readily saturated with oil. The best saturation value is'above 80 percent of the sponge capacity. The pores formed in the silver sponge produced from the zinc'alloy resulting from treatment at 1150 F. for 48 hours at 4 mm. pressure are interconnected. It is for this reason that impregnation of nearly 100 percent is possible in the silver sponge.

Amsler seizure tests were made in S. A. E. 30 oil (Atlantic Neutral Grade) maintained at a temperature of 250 F. Frictional properties are determined by the load that can be applied tov the bearing without causing a marked increase in the coeilicient of friction.

Ihe two silver Sponges prepared from the silver-zinc and the silver-cadmium alloys show better seizure resistance on smoothly finished shafts than does pure silver and also superior to that of the copper-lead and bronze bearing materials. The sponges from the silver-zinc alloy show excellent seizure resistance against a rough shaft. The sponges from the beta brass have a much lhigher resistance to seizure against the rougher shaft than any of the copper-lead bearing materials.

Units of copper and silver sponges have been tested with the Amsler seizure test. 'I'he silver sponge formed from the silver-zinc alloy has excellent seizure resistance, being considerably superior to pure cast silver against both very smooth and slightly rougher shafts. The sponge from the silver-cadmium alloy also has excellent seizure resistance against the very smooth shaft. The copper sponge from beta brass has greater seizure resistance than copper-lead bearing materials against the rougher shaft. It is extremely remarkable that the sponges showed these excellent anti-friction properties in their porous condition without being impregnated with oils or-special anti-friction metals. The investigation has therefore produced the rather unexpected result that a porous surface produced by evaporation is ideally suited for bearing applications.

In impregnation of copper Sponges with lead the sponges are first reduced by hydrogen and thereafter impregnated with molten lead.

Electrolytic condensers are made of aluminum foil .00025 to .005 inch thick which is placed in contact with an electrolyte which forms a very thin insulating film on the aluminum surface. `'Ihe capacity of the condenser depends upon the microscopic surface area of the aluminum. For this reason aluminum foil is often etched or roughened to increase its capacity. It is contemplated that a high capacity foil can be made by the present process using an alloy foil of aluminum-zinc, aluminum-cadmium or aluminum-cadmium-zinc. Aluminum-zinc alloys when evaporated at elevated temperatures have a tendency to shrink thereby reducing the porosity obtained during evaporation.

It is therefore desirable to select alloying ingredients for aluminum having a low boiling point. Such ingredients are mercury, phosphorus, sulphur, selenium, rubidium, potassium. arsenic, cesium. In the case of selenium it is possible to produce actual alloys by melting or by a diffusion process. Also mercury possesses a definite solubility in aluminum.

The introduction of some of the other elements 'can be better accomplished by passing their vapors iny a non-oxidizing atmosphere over the aluminum.

The aluminum foil is heated in a vacuum according to the process already described to vaporize the zinc, cadmium. or the other volatile ingredients, and leave a micro-porous aluminum sheet, which may also be represented by Figure 5.

In the case of aluminum-zinc alloys the boiling point of aluminum is 1800 C. while that of zinc is 905 C. However, aluminum melts at 660 C. and this melting point is further reduced in an aluminum-zinc alloy. A 12 percent zinc alloy melts at about 600 C. By heating the foil in a comparatively high vacuum to 500 C. the evaporation of the zinc is started. The temperature can then gradually be raised to 600 C. as most of the zinc is eliminated so that the foil is kept below its melting point at all times. K

The porous aluminum foil produced by this method is film-formed and used as electrodes for electroiytic condensers.

Micro-porous metal filters can be made in a similar manner. In this case precious and semiprecious metals are alloyed with zinc or other volatile metal. the alloy rolled into a thin foil preferably .00025 to .050 inch thick and heated to volatilize the low boiling constituent. Gold, platinum or palladium are preferred for highly material after the evaporation process, the re suits are good. The strength of such porous strips, even with a relatively high porosity, such as 30 percent, is in the neighborhood of 15,000- 18,000 p. s. i. When it is realized that the average copper-lead bearings containing 25-30 percent lead have only tensile strength values of 10,000 p. s. i. or less it becomes evident that the strength of the evaporated sponges is greatly superior and permits their use in highly loaded high speed bearing applications.

Steel backing materials may be used as part of the products and processes of this invention.

For bearing purposes, a small amount of microporosity of the bearing surface will form antifriction material retaining pockets. Such surface porosity is produced by first diffusing into pure silver or pure copper, or other similarly suitable material, a certain amount of an ingredient with a low boiling point, for example zinc, and subsequently evaporating this low boiling point, constituent from the surface layer.

A practical application would be a sleeve bearing such as a silver lined steel backed bearing. Such a bearing may be produced in the ordinary manner, using electroplating to attach the silver layer. Then a quantity of zinc or cadmium or other similarly suitable metal may be plated on the silver and diffused therein to a depth of .003- .010 inch. After this diffusion process the bearings are subjected to an evaporation process which removes a large part of the diffused metal, leaving a porous surface, controlled in its porosity within close limits by the amount of metal which is diffused.

It has been determined that -10 percent porosity in the surface layer is sufficient to increase the oiliness or anti-friction characteristic of the bearing as well as the wettability. Such surface porosity formation leaves a strong fatigue resistant backing.

If desired, the cavities of the porous surface may be filled as described herein (column 2, lines 37 and 53). This produces a sort of grid bearing in which the grid pattern is microscopic and most effective.

This process may also be used by diffusing metals into the top layer of a bearing or into condenser foil and removing the diffused metal by an etching process. This is particularly applicable when the bearing surface layer or the condenser foil is aluminum since the high temperatures required for the evaporation process cause high shrinkage and resintering thereby closing up the voids produced by the evaporation process.

In vapor diffusion alloying between the copper strip and zinc vapor may be done in a small vertical pressure chamber constructed by welding a plate onto one end of a piece of steel pipe, the other end being closed by a standard screw cap. A quantity of zinc is placed in the bottom of this chamber, and specimens are suspended vertically over it from a washer locked between the top of the tube and the cap. After assembly, the chamber is evacuated to remove the oxygen, being twice vented to hydrogen, and finally being vented to a mixture of 1/4 hydrogen and 3/4 nitrogen. which is used as an atmosphere to prevent oxidation. The chamber is sealed and placed in a furnace at a temperature of 1380 F. for a period of two hours to allow diffusion to take place.

Ailoying between the zinc vapor andthe copper occurs over the full length of the copper strip.

Vapor diffusion of zinc into copper has been successful in producing a brass layer on copper. A thin layer of alpha brass is produced adjacent to the copper and a relatively thick layer of beta brass is formed. Subsequent evaporation of the combination produces a porous surface with a solid copper backing.

impregnation and evaporation experimental apparatus for this method is shown in Figure 6. The chamber 4| shown is -a six-inch length of standard 21/2-inch pipe, welded to a V4 inch, hot rolled steel-base plate. The chamber has a flange 43 welded at the top and is recessed at 4l to receive a copper gasket M. The cover Il is hot-rolled steel, with a boss 4I to match the gasket recess 46, which insures the application of pressure to the gasket. Special flange bolts 4I of alloy steel are used. The purpose of the orifice 49 is to reduce the loss of zinc from the chamber. The charcoal cover 50 prevents the oxidation of the work piece 5I and the zinc 52.

A graphite cup 53 fits the bottom of the chamber, and this prevents the solution of iron in the alloy. The work units are supported from the cover l5. A splash guard 5l over the alloy prevents splashing onto the tube.

Vacuum evaporations may be made from this same chamber by removing the graphite cup and alloy.

Soft copper tubing, two inches in diameter and of 0.063 inch wall thickness is used. The units are degreased and dipped in sodium cyanide solution to insure a clean surface for the reaction.

The work units are positioned in the top of the chamber which is sealed and tested. The chamber is then purged twice with hydrogen to insure a reduced surface on the copper when hot. This chamber is` placed in a furnace at 1380 F. and allowed to come up to temperature. It is flushed once with hydrogen when furnace temperature has almost been reached. When finally hot, it is evacuated and maintained under vacuum for the duration of the impregnation. The alloy is covered with a loose layer of charcoal to reduce the oxygen admitted, and the chamber removed from the furnace and cooled while holding at atmospheric pressure.

The vapor pressure of the zinc in the chamber is decreased by dilution to avoid the production of excess brass. This dilution is accomplished by using tin as provided in a zinc-tin alloy.

A creep-resistant steel is used in the cover bolts to avoid leakage and consequent oxidation. A metal splash guard is provided in the chamber to stop splatter from the alloy. This does not interfere greatly with gas flow.

Figure 8 shows a bearing with backing layer 3l, bearing layer 39, and porous surface 40.

In making bearings by this method, shells of copper are used, having zinc diffused into the surface at elevated temperatures. This is done in an evacuated chamber to produce uniform coatings. The zinc vapor is obtained from a zinc alloy to control the composition of the copper-zinc layer.

Very satisfactory coatings are produced. Alpha invention broadly within 11 or beta brass layers 0.008 to 0.014 inch thick are obtained.

Instead of producing a surface brass layer by diffusion it is also contemplated to electroplate brass directly on the backing such as copper. and subsequently evaporate the zinc from the electroplated brass. a

Instead of electroplating an alloy of copper and zinc such as brass, it is further contemplated to electroplate thin alloy layers such as silverzinc, silver-cadmium and copper-cadmium onto .a bearing liner, and evaporate the volatile ingredient thereby producing ideal bearing surfaces.

It is an advantage of such alloy plating methods that they can be applied to practically iinished bearing shells. It is a further advantage that electroplating permits the formation of alloy compositions which are diiilcult to obtain by melting because of the possibility of codeposition of otherwise immisci-ble ingredients. In electroplating of alloys supersaturated solutions which are unstable may be formed resulting in an extremely iin'e distribution of the ingredients forced into such unstable solid solutions.

Porous bodies, surfaces, and structures according to this invention may also be used in electrical and mechanical elements which are subject to frictional loads, as the load bearing portion or surface. Such elements as sliding contacts, commutator segments, gears, cams, gibs, and ways are examples.

N In forming porosity in metallic members according to this invention a layer of two metals of different melting points is first formed by alloying, diffusion or electrodeposltion. An appreciable percentage of the lower boiling point metal is then removed by vaporization, or an appreciable percentage of one of the metals is removed by etching with a suitable etching agent.

It is within the scope of this invention to join two metals of substantially the same .boiling points or with both boiling points high or low, or in any advantageous arrangement and thereafter remove an appreciable amount of one of the metals by etching with a suitable etching agent.

While specific embodiments of the invention have been described, it is intended to cover the the spirit and scope of the appended claims.

4What is claimed is:

1. The method of providing a porous bearing surface comprising forming a bearing overlay metal with a backing of ferrous metal and a facing of an alloy of metal selected from the group consisting of copper, silver, gold, aluminum, platinum and palladium and a low boiling metal selected from the group consisting of zinc and cadmium, rolling said overlay metal into a sheet metal body, then heating the body to the vaporization temperature of the low boiling metal but below the melting point of the alloy to remove an appreciable percentage of said low boiling metal from said facing and leave a bearing facing layer of fine porous structure.

2. The method of providinga metallic member having a porous surface comprising forming an overlay metal with a backing of ferrous metal and a facing of an alloy of metal selected from the group consisting of copper, silver, gold, aluminum, platinum and palladium with a low boiling metal selected from the group consisting of zinc and cadmium. rolling said overlay metal into a sheet metal body, heating the body to the vaporization temperature of the boiling metal but below the melting point of the alloy to remove an appreciable percentage of said low boiling metal from said facing and leave a. facing layer of flne porous structure, and then impregnating said facing with a lubricant.

3. The method of forming a porous surface upon an object which comprises forming a layer of metal selected from the group consisting of silver, copper, gold, platinum, and palladium upon an object of ferrous metal, diffusing a low boiling point material selected from the group consisting of zinc and cadmium into said layer, and heating said object to a temperature between the boiling point of said metal and said low boiling point material to evaporate said low boiling point material from the surface of said layer and form a porous structure.

4. The method of forming a porous surface upon an object which comprises forming a layer of metal selected from the group consisting of silver, copper, gold, platinum, and palladium upon an object of ferrous metal, diffusing a low boiling point material selected from the group consisting of zinc and cadmium into said layer, heating said object to a temperature between the boiling point of said metal and said low boiling point material to evaporate said low boiling point material from the surface of said layer and form a porous structure, and impregnating said porous structure with lubricating material.

FRANZ R. HENSEL.

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